To gain a better understanding of the pathogenesis of human immunodeficiency virus (HIV), the investigator proposes to use infection of macaques with a closely related simian virus isolated from sooty mangabey monkeys (SIVsmm). Dr. Fultz has identified a unique system for dissecting the evolution of an acutely lethal virus, SIVsmmPBj14, which was isolated from a pig-tailed macaque 14 months after inoculation with SIVsmm9, a prototype virus that induces AIDS-like disease. The applicant will use sequential isolates obtained at regular intervals between inoculation of SIVsmm9 and recovery of SIVsmmPBj14. The investigator will then attempt to define, at the biological and molecular levels, the evolution of the acutely lethal prototype and eight in vitro biologic properties that differ between SIVsmm9 and SIVsmmPBj14. This will be accomplished by characterization of the sequential SIVsmmPBj14 isolates for pathogenicity in pig-tailed macaques and for each of the eight biologic properties. In addition, some sequential isolates will be molecularly cloned to generate infectious viruses that reproduce all the phenotypes of the isolate from which they were generated. Because of distinct differences between SIVsmm9 and SIVsmmPBj14 in the LTR (long terminal repeat) and env (specifies viral membrane glycoproteins) and nef (negative regulatory factor [formerly 3' ORF]) genes, the applicant proposes to amplify relevant areas of the genome from all isolates by polymerase chain reaction (PCR). She will then sequence these to correlate acquisition of specific biologic properties with changes in specific regions of the genome. This should be possible since some of the unique properties of SIVsmmPBj14 are known to segregate. The in vivo relevance of the genomic quasispecies will be assessed using PCR and sequence analysis directly on DNA isolated from tissues of infected animals. Finally, to gain insight into virus-lymphocyte interactions as they relate to disease, the investigator proposes to determine whether SIVsmmPBj14 replicates in resting lymphocytes and to define the mechanism(s) by which this variant activates and induces lymphocytes to proliferate. Dr. Fultz will attempt to detect infection of lymphocytes at the single cell level, to assess the role of cytokines and the effects of metabolic inhibitors and to determine the specificity of induction of proliferation through the use of blocking agents and enriched cell populations.